Teletext systems have been proposed for transmitting information to users having television receivers equipped with decoders which store selected "pages" of the information and display the stored information in the form of alpha-numeric or graphical character symbols on the screen of the television receiver. The information is conveyed by means of pulse code modulation (PCM) of the luminance signal during a selected line (or lines) of the vertical interval so as not to be visible to the user when viewing regular television programming. Such systems are "one-way" in the sense that the information is continuously transmitted and the user identifies portions of the transmission to be stored and displayed. Videotext systems are similar but employ two-way transmissions via telephone lines or other suitable data links to communicate the user's information selection code to the information provider. In both types of systems the user must create and communicate a data selection code to the system.
To facilitate user selection of the information to be displayed in teletext systems, it has been proposed to transmit so-called "menus" of information available in the teletext system databse. A menu may include a listing of available "pages" or topics of information within the system. Identification of specific user selectable items on a menu may be provided by means of specific words or characters. For example, a magazine page might be identified by a specific page number and a command function might be identified by a specific word (e.g., "NEXT" meaning "fetch the next page", "MORE", etc.).
It has been suggested that user selection of teletext menu items be provided by means of a keyboard having keys with labels or symbols corresponding to the "committed" vocabulary of the teletext service. The vocabulary might include numerical digits for page entry, numerical digits followed by a special prefix or suffix (e.g., a pound sign #) for menu selection items and numerical digits followed by a asterisk (*) to denote magazine numbers. Control or command words under consideration include "NEXT" and "MORE". Videotext systems, having direct communication with the information provider, may be expected to have a more extensive vocabulary.
Control of a teletext decoder using a keyboard and the foregoing control vocabulary could be provided, illustratively, by fourteen (14) keys, ten for the numbers 0-9, two for the pound (#) and asterisk (*) symbols and two more for the "commands" NEXT and MORE. Some manufacturers, however, might elect to provide a fewer number of keys (e.g., 12 keys) by omitting the NEXT and MORE keys and letting those commands be represented by double keystrokes (e.g., ** or ##). On the other hand, other manufacturers might elect to provide more than fourteen keys to allow for additional control functions such as `ERASE`, `INDEX`, `CAPTION`, `BACK`, etc.